You can't quite do what you want, but I think we can get you a step closer
to what you need.

Let's start with the difference between master slides and layouts... Layouts
let you define which elements are on a slide of a particular layout. Master
slides are (technically) the slide at the top of a chunk of layouts. They
are the starting point for all the layouts that are built from them. So -
no, they aren't really like cascading style sheets. They are more like
layers of tissue paper, each with a different part of the slide
design/layout. Items on the master slide are always behind all other items.
Items on the layout are between items on the master slide and the regular
slide. The exception is that placeholders on the layout can be brought in
front of objects on a slide.

And this is where you may be able to get closer to what you want with your
buttons. If you set up a series of small placeholders that are formatted the
way you want them to be, you can use these as buttons on your slides. When
you play the slides, you will only see the buttons which have content in
them (either a picture or text - even a single space will do). You can set
up hyperlinks from the placeholders to other slides and locations or you can
use them to trigger animations.

Is that clearer? I think I know what you are attempting to do with your
presentations, I am not sure that what you want to use the layouts for will
get you where you want to go.

Echo - if you are out there today, can you help me explain this? You know
where I am going and what I am trying to say here. And what I wish we had
working... but that is ancient history now.